Colors are off, outside of blender
Update September 20, 2018 - Filmic now Supports Apple DCI-P3!

You can download the branch for testing here. It will get merged into the main branch after a little time for testing.

Original Answer

The issue is that Blender isn't designed for your MacBook Pro 2016 nor any model beyond it. I have brought up this point many times, but no one cares.

The reason is that the RGB coloured lights in your MacBook Pro are different to the sRGB standard coloured lights. Blender's camera rendering transforms (displays and views) are designed solely for REC.709 based displays. Blender simply sends values that are encoded assuming REC.709 flashlights at the display, and when those lights aren't REC.709 colours, the results are completely broken.

macOS assumes imagery without an ICC profile attached are sRGB. This is close to how the imagery is designed to look, when viewed through Preview, as Preview will properly transform the imagery to the DCI-P3 based lights in your display.

So what is the problem, exactly? Blender is showing you incorrect colours while working within it. In order to properly get a 1:1 WYSIWYG within Blender, the developers would need to integrate a series of camera rendering transforms designed for your display, as well as fix Blender's areas where it isn't properly colour managed.

With a little effort, you could test the values in the development branch Filmic Blender. Sadly, I haven't had time to update the entire configuration to support Apple P3 displays, but the basic values are in there for testing.